This invention is concerned with analog-to-digital converters. More particularly, it is concerned with oversampling delta-sigma analog-to-digital converters which are suitable for performing single-cycle conversions.
Although real world signals are analog, it is often desirable to convert them into the digital domain using analog-to-digital (A/D) converters. Circuit designers are motivated to perform this conversion because of the efficient methods currently available for the transmission, storage and manipulation of digital signals. A digital representation of an audio signal, for example, allows a CD player to achieve virtually error free storage using optical discs. The need for complicated signal processing may also necessitate A/D conversion because such signal processing is only feasible in the digital domain using either digital computers or special purpose digital signal processors. Signal processing in the digital domain is also particularly useful in such areas as biomedical applications to provide the required accuracy for such tasks as magnetic resonance imaging (MRI).
Oversampling methods have recently become popular in A/D converters because they avoid many of the difficulties encountered with traditional methods for analog-to-digital conversion. This is because such converters have certain attributes that are difficult to implement on integrated circuit devices. Foremost among these is the use of analog filters and the need for high-precision analog circuitry that is vulnerable to noise and interference. The virtue of traditional conversion techniques is the ability to use a relatively low sampling frequency, usually the Nyquist rate of the input signal (i.e., twice the signal bandwidth).
Traditional A/D converter systems require low-pass anti-aliasing filters at their input to attenuate the magnitude of high-frequency noise and out-of-band components of the signal that alias in-band when sampled at the Nyquist rate. The frequency characteristics of such filters are determined by the precision of the analog components with which they are made.
On the other hand, oversampling A/D converters can use simple and relatively high-tolerance analog components, but require fast and fairly complex digital signal processing stages. They modulate the analog input into a simple digital code, usually single-bit words, at a frequency much higher than the Nyquist rate. These modulators operate by trading off resolution in amplitude for resolution in time such that imprecise analog circuits may be used. The use of high-frequency modulation can eliminate the need for abrupt cutoffs in the analog anti-aliasing filters at the input to the A/D converter. A digital low-pass filter can smooth the output of the digital modulator, attenuating noise, interference, and high-frequency components of the signal before they can alias into the signal band when the signal is re-sampled at the Nyquist rate. Another digital filter may be used to decimate the single-bit code to a high-word-rate digital pulse code modulated signal at the output.
Oversampling A/D converters make extensive use of digital processing, taking advantage of the fact that integrated circuit technology is better suited for providing fast digital circuits than for providing precise analog circuits. Because their sampling rate usually needs to be substantially higher than the Nyquist rate, conventional oversampling A/D converters are generally used in medium-speed digital applications such as digital audio, digital telephony and data communications. Such applications require the continuous production of A/D conversion results from a single input source. As a result, a well-defined settling time is of little importance to conventional A/D converters. Furthermore, power consumption in conventional oversampling A/D converters tends to be relatively high because the converter circuitry must remain constantly activated to produce continuous conversion results.
Nevertheless, due to their exceptional resolution and accuracy, oversampling A/D converters are also desirable for such tasks as data acquisition, industrial control and test and measurement instrumentation. In many such applications, only a single conversion result from a given input source is desired. For example, an analog input signal may be selected from a number of uncorrelated distinct sources by an input multiplexer or may be sampled at uncorrelated time intervals depending upon the desired function. In such applications, the A/D converter makes only one conversion for a given input signal before a new input signal is provided for conversion. Therefore, in single conversion or "single-cycle conversion" applications, output data oversampling and strong time correlation between successive conversion results is of little importance.
Conventional oversampling A/D converters are not well suited for performing single-cycle A/D conversions because they are designed to repeatedly sample a given analog input signal for an extended period of time to produce multiple conversion results at a rate higher than twice the input signal bandwidth. Such A/D converters have complex digital filtering stages and poorly defined settling times which tends to make input multiplexing difficult. In addition, because conventional oversampling A/D converters are typically designed to operate continuously for extended periods of time, they contain no power saving circuitry for turning off portions of the converter when not in use, a condition that occurs frequently in single-cycle operation.
In view of the foregoing, it would be therefore desirable to provide an oversampling A/D converter suitable for providing single-cycle A/D conversions.
It would also be desirable to provide an oversampling A/D converter that included circuitry for selectively turning on and off portions of the converter circuitry to minimize power consumption when conversions are not required.
It would be also desirable to provide an oversampling A/D converter that included digital filter circuitry of reduced complexity.
It would be additionally desirable to provide an oversampling A/D converter that has a well defined settling time to facilitate input multiplexing.